Everything, therefore, takes place as if, by the fact of
electrification, its capacity for kinetic energy and its material mass
had been increased by a certain constant quantity. To the ordinary
mass may be added, if you will, an electromagnetic mass.
This is the state of things so long as the speed of the translation of
the particle is not very great, but they are no longer quite the same
when this particle is animated with a movement whose rapidity becomes
comparable to that with which light is propagated.
The magnetic field created is then no longer a field in repose, but
its energy depends, in a complicated manner, on the velocity, and the
apparent increase in the mass of the particle itself becomes a
function of the velocity. More than this, this increase may not be the
same for the same velocity, but varies according to whether the
acceleration is parallel with or perpendicular to the direction of
this velocity. In other words, there seems to be a longitudinal; and a
transversal mass which need not be the same.
All these results would persist even if the material mass were very
small relatively to the electromagnetic mass; and the electron
possesses some inertia even if its ordinary mass becomes slighter and
slighter.
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